The present invention relates to a hydraulic fluid control apparatus of the type used, for example, in a braking system for an aircraft.
The high cost of aircraft development means that development programs for new aircraft need high production numbers and/or selling price to justify investment levels required. In the aviation industry, there are many niche applications for small numbers of aircraft, making investment in a new aircraft difficult to justify. However, aging aircraft that currently fill many of these niche functions are operating with equipment that can be obsolete or inefficient, because of the outdated technology employed. Such equipment can include engines and ancillary equipment, avionics, landing gear and control systems.
An increasing trend in aviation is the updating of proven airframe designs with modern equipment to provide an updated, more efficient aircraft that utilises current technology without the cost associated with development of a completely new aircraft. Some older airframes that are now being refurbished and re-equipped for a new life span were originally designed for braking systems employing sintered friction materials. These aircraft are now being equipped with modern braking systems. An example of such an aircraft is the Nimrod MRA4 built by BAE Systems Limited of Great Britain. However, one drawback of this approach is the accommodation of the performance parameters of new modern systems by the existing airframe, in particular the torque generated by a braking system.
In this respect, the modern aircraft braking systems are designed around the use of carbon-carbon composite friction materials that have a significantly higher peak torque during a braking cycle than sintered materials. Consequently, if the brake torque builds too quickly and/or exceeds certain threshold values when the brake is applied, it is possible to cause damage to the airframe.
Brake control on aircraft employing such airframes refurbished with braking systems using carbon-carbon composite friction materials has been carried out by the use of complex systems within hydro-mechanical brake pressure control valves to control brake torque and avoid damage to the airframe. Typically, a pressure reducing valve, under the control of a utilities systems management system, feeds hydraulic fluid under pressure to brake metering valves, anti-skid valves and hydraulic fuses downstream of the pressure reducing valve. The pressure reducing valve builds a pressure profile in response to a brake signal received by the pressure reducing valve, the pressure to the brakes being increased over a pre-determined time from a starting pressure to some peak value during braking until the pressure is then released after the required brake application has been completed. The functionality of the pressure reducing valve is provided mechanically by, for example, the use of a restrictor to limit the flow of hydraulic fluid and an accumulator, a build-up of pressure being dependent on the restrictor size and accumulator volume.